#0: Enable unity for sublibraries (#16450)

### Problem description
Clean builds slowed down after splitting ttnn in sub-libraries. 

### What's changed
Enabled unity build for ttnn sub-libraries. Reduced clean build time to
be similar to the original build time before splitting ttnn into
sub-libraries.

### Checklist
- [x] Post commit CI
[passes](https://github.com/tenstorrent/tt-metal/actions/runs/12630837775)

ttnn/CMakeLists.txt

@@ -734,6 +734,7 @@ function(add_ttnn_sublibrary SUBLIBRARY_NAME)
     else()
         add_library(${SUBLIBRARY_NAME} OBJECT ${ARGN})
     endif()
+    TT_ENABLE_UNITY_BUILD(${SUBLIBRARY_NAME})
     if(WITH_PYTHON_BINDINGS)
         target_compile_definitions(${SUBLIBRARY_NAME} PUBLIC TTNN_WITH_PYTHON_BINDINGS=1)
     else()

ttnn/cpp/ttnn/operations/transformer/concatenate_heads/concatenate_heads.hpp

@@ -2,6 +2,8 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
+#pragma once
+
 #include "ttnn/decorators.hpp"
 
 namespace ttnn {